1. Technical Field
This disclosure relates generally to an engine control system for automotive vehicles, and more particularly to such a control system designed as an engine speed predicting system to predict the speed of an engine while it is dropping after fuel stops being burned in the engine.
2. Background Art
Engine control systems for automotive vehicles are known which are equipped with an idle-stop feature (also called an automatic engine stop/restart system) designed to detect driver's action, such as accelerating or braking, and start or stop an engine of the vehicle automatically in order to reduce the consumption of fuel in the engine.
There has also been proposed a technique to restart the engine as quickly as possible without waiting for an output shaft of the engine to stop rotating in response to an engine restart request while the speed of the engine is decreasing. For instance, Japanese Patent First Publication No. 2011-140938 teaches such an engine restart system. This system also works to calculate the speed of the engine while it is dropping to mathematically project a future speed of the engine after a lapse of time required to bring a pinion of a starter into engagement with a ring gear coupled to an output shaft of the engine and to control an operation of the starter as a function of the projected speed of the engine. The publication also teaches taking into consideration the fact that the speed of the engine usually pulsates in a period of time where the speed of the engine drops following an automatic stop of the engine to calculate the speed of the engine based on a loss of energy in the engine occurring in a previous calculation cycle.
Usually, once the speed of the engine decreases to approximately zero after automatic stop of the engine, it causes the piston of the engine to fail to pass the top dead center, so that the rotation of the engine will be reversed. Subsequently, the engine continues to rotate alternately in forward and reverse directions and then finally stops. It is usually difficult to predict the time when an engine restart request should be made while the speed of the engine is dropping. For instance, the engine restart request is sometimes issued while the output shaft of the engine is rotating in the reverse direction. It is advisable that the engine be restarted as quickly as possible in response to the engine restart request to minimize a lag in starting the engine.
There is a possibility that bringing the pinion into contact with the ring gear to achieve engagement therebetween while the output shaft of the engine is rotating in the reverse direction will result in increased mechanical noise or wear of the pinion or the ring gear. It is, therefore, necessary to achieve the engagement between the pinion and the ring gear in a range where the speed of the engine will not result in serious noise or wear. The achievement of engagement between the pinion and the ring gear in such a range requires accurate prediction of the speed of the engine. The engine restart system, as taught in the above publication, is capable of determining the speed of the engine accurately until it drops to zero for the first time after the burning of fuel in the engine is stopped, in other words, while the engine is rotating in the forward direction, but however, may make a great error in calculating the speed of the engine while the output shaft of the engine is rotating in the reverse direction.